1. Field of the Invention
The present invention relates to a proton conductive material and a method for manufacturing same. The present invention also relates to a membrane-electrode assembly containing the proton conductive material and suitable for a solid polymer electrolyte fuel cell.
2. Description of the Related Art
Fuel cells directly convert chemical energy into electric energy by supplying a fuel and an oxidizing agent to two electrodes that are electrically connected and electrochemically inducing oxidation of the fuel. By contrast with thermal power generation, fuel cells are not affected by the limitations of Carnot cycle and, therefore, demonstrate a high energy conversion efficiency. A fuel cell is usually configured by stacking a plurality of unit cells containing as a basic structure a membrane-electrode assembly in which an electrolyte membrane is sandwiched between a pair of electrodes. Among such fuel cells, fuel cells of a solid polymer electrolyte type that use a solid polymer electrolyte membrane as the electrolyte membrane attracted attention as power sources, in particular, for portable devices and movable bodies, because such fuel cells have a number of advantages including the easiness of miniaturization and operability at a low temperature.
In a fuel cell of a solid polymer electrolyte type, when hydrogen is used as a fuel, a reaction represented by Equation (1) proceeds at an anode (fuel electrode).H2→2H++2e−  (1)Electrons generated according to Equation (1) perform a work in an external load via an external circuit and then reach a cathode (oxidizing agent electrode). Protons generated according to Equation (1) move by electroosmosis from the anode to the cathode inside the solid polymer electrolyte membrane in a state of hydration with water.
Further, when oxygen is used as an oxidizing agent, a reaction represented by Equation (2) proceeds at the cathode.2H++(½)O2+2e−→H2O  (2)
Water generated at the cathode mainly passes through a gas diffusion layer and is discharged to the outside. Thus, fuel cells are nonhazardous power generating devices producing no wastes other than water.
A polymer electrolyte membrane that may operate in a temperature range of fuel cells of a solid polymer electrolyte membrane type that are usually used is composed of a proton conductive material of an organic polymer type that has a polymer in a basic skeleton or main chain. Problems associated with such proton conductive materials include dimensional changes such as expansion and contraction of the membrane during water absorption and desorption and the occurrence of heat-induced creep or thermal shrinkage. In the operation environment of fuel cells, the water and heat balance is known to change frequently due to a load or external environment, and because dimensional changes of the membrane caused by such changes shorten the electrolyte service life, it becomes a serious problem that may not be in principle resolved with the presently available proton conductive materials of an organic polymer type.
With the foregoing in view, proton conductive materials of an inorganic polymer type are presently being actively developed. Japanese Patent Application Publication No. 2003-281931 (JP-A-2003-281931) discloses that using a mechanical grinding method with respect to a crystalline metal phosphate such as zirconium phosphate makes it possible to attain a high proton conductivity in a proton conductive material containing such a phosphate. Thus, it is shown that a macroordered structure is destroyed in mechanical grinding, and a microordered structure is ensured, while the material is being processed into a powder, thereby making it possible to maintain a high proton conductivity in a high-temperature dry environment.
Further, Japanese Patent Application Publication No. 2004-55181 (JP-A-2004-55181) discloses that by adding a metal phosphate to a phosphosilicate gel or silica gel, it is possible to obtain a high proton conductivity in a proton conductive material. Thus, it is indicated that by adding a metal phosphate having a structure retaining moisture, which is a proton conduction carrier, it is possible to maintain a high proton conductivity in a high-temperature dry environment.
Further, Japanese Patent Application Publication No. 2006-147478 (JP-A-2006-147478) discloses that by adding zirconium phosphate to a polymer compound having ion conductivity, it is possible to attain a high proton conductivity in a proton conductive material. Thus, it is shown that by combining an inorganic substance demonstrating a high proton conductivity at a high temperature with an organic polymer that has a drawback of the proton conductivity decreasing under a high-temperature dry environment, it is possible to overcome this drawback.
An attempt has also been made to develop a new proton conductive material of an organic polymer type. Japanese Patent Application Publication No. 2002-193861 (JP-A-2002-193861) discloses that a fullerene polymer having proton conductivity may be synthesized by introducing a sulfonic acid group into a fullerene, which is a carbon allotrope, and crosslinking the fullerene derivatives with a biphenyl or the like.
Among the aforementioned reference documents, JP-A-2003-281931, JP-A-2004-55181, and JP-A-2006-147478 use metal phosphates, which are inorganic materials. Usually, where an attempt is made to increase the proton conduction capacity of inorganic materials by adding proton conductive groups, hydration proceeds and the material becomes a liquid with high flowability. As a result, the shape retaining ability is extremely poor. Therefore, a limitation is placed on the increase of proton conduction ability of metal phosphates. Further, in the configuration described in JP-A-2002-193861, the number of proton conductive groups that may be directly introduced onto one fullerene molecule, which consists of a limited number of carbon atoms, is obviously limited. Therefore, in this case, too, a limitation is placed on the increase of proton conduction capacity.
Further, in the case of fuel cells of a solid polymer electrolyte type, the fuel and oxidizing agent are usually continuously supplied in a gaseous state (fuel gas, oxidizing agent gas) into the fuel cell. These gases are introduced as far as a three-phase interface that is a contact surface of catalyst particles supported on a support that is an electric conductor and a polymer electrolyte that ensures ion conduction paths, and the above-described reactions represented by Equations (1), (2) proceed. Therefore, porous electrodes in which a polymer electrolyte is homogeneously mixed with catalyst particles are known to be usually used as the electrodes of fuel cells.
However, because catalyst particles, the polymer electrolyte, and the support that is an electric conductor are homogeneously dispersed to a high degree in the electrode, the gas diffusion paths are uniformly narrow and the discharge paths for discharging the generated water from the reaction field to the outside of the system are difficult to ensure. After a fuel cell has been operated for a long time, the gas diffusion paths become even narrower due to accumulation of the generated water, thereby decreasing gas diffusability.
Adjusting the mass ratio of the proton conductive material and electric conductor in the catalyst has heretofore been suggested as means for resolving the problems associated with both the decrease in gas diffusability and the decrease in dischargeability of generated water. Japanese Patent Application Publication No. 2007-80694 (JP-A-2007-80694) discloses that a catalyst layer with excellent initial performance may be provided when the mass ratio of the proton conductive material to the electric conductor is equal to or higher than 0.6 and lower than 0.8.
Alternatively, an attempt has been made to adjust the discharge amount of generated water by providing a hydrophilic layer between a polymer electrolyte membrane and a catalyst layer. Japanese Patent Application Publication No. 2005-25974 (JP-A-2005-25974) discloses a configuration in which a layer composed of a proton conductive material with a hydrophility higher than that of a catalyst layer is provided between a polymer electrolyte membrane and the catalyst layer, thereby resolving both the problems of decrease in the water content ratio in the polymer electrolyte membrane and the problem of decrease in power generation performance caused by excessive amount of moisture in the catalyst layer.
However, in the configurations described both in JP-A-2005-25974 and in JP-A-2007-80694, an organic polymer electrolyte resin having a sulfonic acid group is usually used in the proton conductive material. An organic polymer electrolyte resin having a sulfonic acid group in a structure has poor ability of retaining water for a long time and, therefore, the membrane-electrode assembly deteriorates significantly when the fuel cell is operated under low-humidity conditions. Further, because the organic polymer electrolyte resin is uniformly dispersed in the catalyst layer or hydrophilic layer in the configurations described in JP-A-2005-25974 and JP-A-2007-80694, the catalyst layer lacks porosity, thereby causing decrease in gas diffusability and dischargeability of generated water.